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What is a Star Schema? Which Schema is preferable in performance oriented way? Why?



                                                                                                                                                                      
Scenario:

What is a Star Schema? Which Schema is preferable in performance oriented way? Why?


Solution:
 A Star Schema is composed of 2 kinds of tables, one Fact Table and multiple Dimension Tables.

                        It is called a star schema because the entity-relationship diagram between dimensions and fact tables resembles a star where one fact table is connected to multiple dimensions. The center of the star schema consists of a large fact table and it points towards the dimension tables. The advantage of star schema are slicing down, performance increase and easy understanding of data.


F1act Table contains the actual transactions or values that are being analyzed.
Dimension Tables contain descriptive information about those transactions or values.

à In Star Schemas, Dimension Tables are denormalized tables and Fact Tables are highly   
     normalized.

Star Schema 

 

Star Schema is preferable because less number of joins will result in performance.
Because Dimension Tables are denormalized, there will be no need to go for joins all the time.


Steps in designing Star Schema
• Identify a business process for analysis(like sales).
• Identify measures or facts (sales dollar).
• Identify dimensions for facts(product dimension, location dimension, time dimension, organization dimension).
• List the columns that describe each dimension.(region name, branch name, region name).
• Determine the lowest level of summary in a fact table(sales dollar).
Important aspects of Star Schema & Snow Flake Schema
• In a star schema every dimension will have a primary key.
• In a star schema, a dimension table will not have any parent table.
• Whereas in a snow flake schema, a dimension table will have one or more parent tables.
• Hierarchies for the dimensions are stored in the dimensional table itself in star schema.
• Whereas hierachies are broken into separate tables in snow flake schema. These hierachies helps to drill down the data from topmost hierachies to the lowermost hierarchies.

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